<<the pilot has the right to ignore that instruction and land anyway, on any runway he fancies.>>
And only a complete and utter fool would.... and I've met a few, believe me. Thankfully not during my time at Heathrow!
<<Quick question for all ATC folk - If you give a go around command, does this not automaticaly CANCEL any previous landing clearance ?>>
Yes..
Point to remember in this discussion is that no matter what a pilot thinks he sees out of the window, ATC may be aware of something he can't. Aircraft aren't sent around for fun; there has to be a very, very good reason.